The present invention relates to a method for detecting a fully charged condition of a secondary battery and the fully charged condition detector.
Presently, portable apparatuses that operate on small-sized high-capacity secondary batteries such as notebook-type personal computers and portable telephones are rapidly becoming widespread. Moreover, electric vehicles that move on large-capacity secondary batteries have been put to practical use.
In the use of these secondary batteries, overcharge should be prevented since it is a serious factor that causes shortenings of the battery cycle life. Moreover, shortening the charging time by accurately detecting the fully charged condition of the secondary battery is desirable for users' convenience.
Conventionally, apparatuses using secondary batteries and battery chargers have been provided with means for preventing overcharge by detecting the fully charged condition of secondary battery under charging.
In most of these methods, a voltage, temperature or internal pressure of the battery is measured and the measurement value is compared with a predetermined value to control charging.
Concrete examples will hereinafter be described.
Japanese Laid-open Patent Publication No. Hei 6-133468 proposes a charging method in which during charging of a nickel-metal hydride storage battery, the fully charged condition is detected based on a temperature, internal pressure and voltage of the battery and the charging mode is changed to trickle charge after the battery is detected to be fully charged.
Japanese Laid-open Patent Publication No. Hei 7-240236 proposes a charging control method in which an apparatus (or a battery charger) charges a battery at a gradually increasing charging voltage, and detects an overvoltage of the battery if the battery is loaded for the first time, and from the next time, it charges the battery at a constant charging voltage of the overvoltage value detected in the initial charging.
Japanese Laid-open Patent Publication No. Hei 8-70537 proposes a method in which a terminal voltage change amount .DELTA.V and a temperature change amount .DELTA.T of a battery being charged are detected with a predetermined interval, and the battery is judged to reach a fully charged condition when .DELTA.V tends to decrease and .DELTA.T tends to increase.
Japanese Laid-open Patent Publication No. Hei 7-240234 proposes a charging method in which, while the voltage of a battery under charging being detected periodically and a change amount in the battery voltage from the value in the last measurement being calculated, a charging current is reduced or charging itself is finished when the difference between the change amount and the maximum value of the change amounts calculated in the last cycles becomes equal to or higher than a preset value.
Japanese Laid-open Patent Publication No. Hei 7-27256 proposes a charging control responsive to a change in charging current.
According to this prior art, there is added to an apparatus a function to limit an output current flowing to an external power source, which monitors the charging current per unit time even when the secondary battery is charged while the apparatus is operating, thereby changing the detection value of the battery temperature increase per unit time and calculating a limitation value of charged amount of the battery.
The fully charged condition detection based on the voltage, temperature or internal pressure of the battery as described above have the following problems.
In the detection based on the battery voltage, when the temperature of the battery increases, voltage changes in the last stage of the charging are difficult to catch, so that an accurate detection of the battery's condition is difficult.
In the detection based on the internal pressure, there is required a special structure for the battery to provide a sensor for detecting the internal pressure inside the battery.
The detection based on the battery temperature is low in response to a condition change, since the detection depends on the heat transmission velocity in the battery. Therefore, this method is not suitable for practical use in a battery system where switching between charging and discharging is made in a short time. Further, it is affected by the environmental temperature of the battery.
As a charging method that requires no measurement of the voltage, temperature or internal pressure of the battery to detect whether the battery is fully charged, Japanese Laid-open Patent Publication No. Hei 7-85892 proposes a charging method in which, while measuring acoustic emission signals (hereinafter, referred to as "AE signals") from the battery by using a root mean square circuit (hereinafter, referred to as "RMS circuit"), the charging mode is changed from constant-current charging to constant-voltage charging when the differentiation value of the measurement tends to sharply increase.
In this method, since a change before and after the battery is fully charged is detected, the change is comparatively easy to detect in a system where charging is continuously performed. However, in a case of repeating pulse-like charging, it is difficult to find a point where the change occurs. In addition, when a fully-charged battery is further charged, it is impossible to detect that the battery is overcharged.